The present invention is related to paste compositions containing metal pigments suitable for forming coating compositions, particularly aqueous coating systems. Increasingly stringent environmental regulations have required that coating systems dramatically reduce volatile organic solvent levels. One way to comply with such regulations is to use water in place of the volatile organic solvents previously used.
However, in the area of coating systems utilizing metal pigment particles, aqueous systems present rather formidable difficulties. This is particularly true with respect to aluminum and zinc pigments. Thus, the metal pigment can readily react with water to generate hydrogen gas. The amount of gas generated can produce a safety hazard, creating high pressures within the composition containers. Also, the water reaction substantially diminishes the aesthetic value of metal pigments. The reaction of aluminum pigments with water can be depicted as follows: EQU 2Al+6H.sub.2 O.fwdarw.2Al(OH).sub.3 +3H.sub.2 (g)
Due to the increasing demand for aqueous systems, a number of techniques have been proposed for inhibiting the attack on the pigment particles by water. Unfortunately, most of these techniques have not provided sufficient protection.
One technique that provides inhibiting properties is the passivation of the metal pigment particles with an ionic organic phosphate as disclosed by Williams et al., U.S. Pat. No. 4,565,716. Other techniques involve the use of compounds containing pentavalent vanadium compounds as disclosed in Kondis U.S. Pat. No. 4,693,754, or the use of organic phosphites as disclosed in Kondis et al. U.S. Pat. No. 4,808,231. Another composition of matter utilized to produce an aluminum paste for use in aqueous systems uses nitroparaffin solvents to inhibit the evolution of hydrogen gas. However, through the addition of nitro-containing solvents we have improved upon the passivation of metal pigment particles utilizing organic phosphates, phosphites, or vanadium compounds.